Solar jets are commonly observed with IRIS and the multi wavelength AIA instrument. IRIS spectroscopic and imaging observations of jets reveal bidirectional outflows in transition region lines at the base of the jets implying explosive magnetic reconnection processes ([1]}; [2]}).
Bidirectional outflows in the LOS are detected by the extended wings in chromospheric and transition line profiles ([3]}; [4]}; [2]}). With IRIS the chromospheric C II and Mg II lines are frequently observed not only in the UV bursts but also in the quiet chromosphere as well as in solar flares and jets ([6]}; [7]}). They are optically–thick chromospheric
lines, which need a radiative transfer approach to determine the physical quantities of plasma. The Mg II lines are formed in multi layer atmosphere.
Simulations in the quiet chromosphere has been carried out by several authors ([8]}; [9]}; [10]}; [11]}; [12]}; [6]}, [14]}; [15]}; [16]}). The core of the line is formed just under the transition region (T\(<\)  20,000 K), the wings at the minimum of temperature (T = 5000 K).
IRIS spectral data allow to make many progresses on the plasma diagnostics in flares and shows the blue and redshifts. The blue asymmetry can be explained by down-flowing plasma absorbing the red peak emission and not by strong blueshift emission
([17]}).
Chromospheric response to intense heating, even in the 1D model, is complicated. The shape of the emission line profiles depends sensitively on the physical conditions of the plasma and its dynamics, in particular the plasma flows that arise at the line core formation heights.
They may have symmetrical profiles. Moreover the highest Near Ultraviolet (NUV) continuum enhancements observed in strong flares are most
likely because of the Balmer continuum formed by Hydrogen recombination ([18]}) and consequently flares can be assimilated to white light flares, commonly observed in optical continuum where the energy deposit is localized at the minimum temperature region.
The ratio of IRIS transition region lines is also a good diagnostics for the determination of the plasma density in flares ([19]}). The electron density (N\(_e\) ) in flare ribbons can be enhanced by two orders of magnitude more than in plage region (N\(_e\)  \(>\)  10 \(^{13}\)  cm\(^{-3}\) ).
For multiple flaring kernels, chromospheric lines show a rapidly evolving
double-component structure: an enhanced emission component at rest, and a broad, highly red-shifted
component of comparable intensity. [20]} interpreted such observations by beams penetrating very deep in the atmosphere. The red-shifted components migrate from redshifts towards
the rest wavelength. The electron beams would dissipate their
energy higher, driving an explosive evaporation, and a counterpart condensation is created as a very dense layer.
